A wiring layer in a semiconductor device functions to transmit signals and is typically connected to lower conduction layers via contact plugs. Contact plugs should typically be formed of low-resistivity metals in order to prevent signal delays.
FIGS. 1A-1D are sectional views for illustrating a conventional method of forming a contact plug in a semiconductor device. In FIGS. 1A-1D, reference numeral 1 is a semiconductor substrate, reference numeral 3 is an inter-insulating layer, reference numeral 5 is a wiring layer, reference numerals 7, 7a and 7b are insulating films, reference numeral 8 is a contact hole, reference numerals 9 and 9a are barrier layers and reference numerals 11 and 11a are material layers. Referring to FIG. 1A, a conductive material is deposited on a semiconductor substrate 1 on which an inter-insulating layer 3 is formed, and then patterned to form a wiring layer 5. Next, an insulating material is deposited on the semiconductor substrate 1 on which the wiring layer 5 is formed, thereby forming the insulating film 7. The wiring layer 5 may be formed of a metal, e.g., aluminum (Al). The insulating film 7 has depressed portions which conform to the structure of the wiring layer 5.
Referring now to FIG. 1B, the insulating film 7 undergoes a chemical and mechanical polishing (CMP) process to form a planarized insulating film 7a. At this time, the insulating film 7a can be formed to a predetermined thickness by controlling the time required for the polishing process. Thereafter, a cleaning process such as a spin scrubbing method is performed to remove particles generated during the polishing process.
Referring now to FIG. 1C, the insulating film 7a is etched using photolithography to expose the surface of the wiring layer 5 and form contact holes 8. Titanium (Ti) and titanium nitride (TiN) are sequentially deposited in the contact holes 8 to form a barrier layer 9 having a titanium nitride (TiN)/titanium (Ti) structure. Then, tungsten (W) is deposited on the entire surface of the semiconductor substrate 1 on which the barrier layer 9 is formed, thereby forming the material layer 11. The titanium reduces the contact resistance between the tungsten as the component material of the material layer 11 and the aluminum as that of the wiring layer 5. The titanium nitride also improves adhesion of the tungsten.
Referring to FIG. 1D, the material layer 11 and the barrier layer 9 undergo a chemical and mechanical polishing (CMP) process until an insulating film 7b is exposed. Accordingly, a plurality of contact plugs comprised of a material layer 11a and a barrier layer 9a are formed in the contact hole 8. As described above, the CMP process is carried out two times, after the deposition of the insulating film 7 and after the deposition of the material layer 11. Unfortunately, the use of two polishing steps complicates the process for forming contact plugs.